1. Field of the Invention
This invention relates generally to an apparatus for reproducing an audio or other information signal recorded on a record disc as a pulse code modulation (PCM) in a run length limited code, and more particularly is directed to a servo system for controlling the rotation of the record disc in such reproducing or playback apparatus.
2. Description of the Prior Art
It is known to provide a digital audio record disc similar to a so-called video record disc, and in which a PCM audio signal is recorded as a series of pits or recesses arranged in a spiral track on the record disc. For reproducing or playing-back such digital audio record discs, it has been proposed to provide playback apparatus of various types, such as, optical, electrostatic-capacity or other types. A digital audio record disc may have a PCM signal recorded thereon for playback or reproducing while the disc is rotated at a constant angular velocity, or while the disc is rotated at a constant linear velocity at the location where the transducer or pick-up is scanning the record disc for detecting the recorded signal. In order to maximize the recording density, it is preferable that the digital audio signal be recorded on the disc for playback at a constant linear velocity.
In the case where the audio PCM signal is recorded in a spiral track on the record disc, the transducer or pick-up for reproducing the recorded signal is moved radially in respect to the rotational center of the disc as the reproducing operation proceeds and, therefore, the angular velocity at which the record disc is rotated has to be varied with time to ensure the maintenance of the desired constant linear velocity at the location where the transducer or pick-up scans or reproduces the signal recorded in the record disc. For varying the angular velocity at which the record disc is driven so as to maintain a constant linear velocity at the location where the transducer or pick-up scans the disc, it has been proposed to couple a potentiometer with the pick-up so that the position of the movable tap of the potentiometer changes with changes in the radial distance of the pick-up from the center of the rotated disc. Since the required angular velocity or rotational speed of the record disc varies inversely with the distance from the pick-up to the center of rotation of the record disc, a detected output can be obtained from the potentiometer and fed to a voltage divider through which a control signal is obtained for controlling the speed of the motor by which the record disc is driven. However, in the foregoing arrangement, the potentiometer acting as a pick-up position detector and the voltage divider for providing the control signal are relatively expensive and complicate the construction of the apparatus.
In producing a PCM audio record disc of the optical type, an original or master disc is initially produced through the use of a laser beam which is optically modulated by a signal to be recorded for providing pits or recesses spaced apart along a record track on the master disc in response to the level "1" or "0" of the signal being recorded. Thereafter, in a record pressing process similar to that employed for the production of conventional analog audio record discs, successive digital or PCM audio discs are replicated from the original or master disc. However, by reason of conditions encountered during the production of the master disc or during the replication of the PCM audio record discs from the master disc, so-called asymmetry may arise, that is, the sizes of the pits may be changed uniformly so as to be, either all increased or all decreased. Thus, if a signal to be recorded has an ON-OFF ratio of 50%, the corresponding signal, when reproduced from a PCM audio record disc which has encountered asymmetry in its production, will have an ON-OFF ratio other than 50%. In such case, when the reproduced signal is converted to a pulse signal by a waveform converting circuit in the reproducing or playback apparatus, the pulse width of the reproduced signal will be different from that of the corresponding signal which was recorded, whereby the reproduced data will not be correctly demodulated or further processed and problems will result. In order to avoid such problems, it has been proposed to employ, as the waveform converting circuit, a comparator which receives the reproduced signal and compares the latter with a reference voltage or threshold level that is adjustable manually for correcting asymmetry. However, such manual adjustment of the threshold level is obviously troublesome.
In order to eliminate asymmetry without the need for manual adjustment of a threshold level, it has been suggested, for example, as described in detail in each of U.S. Pat. Nos. 4,397,011 and 4,418,406, and having a common assignee herewith, to vary the threshold level automatically in response to data included in the reproduced signal derived from the record disc. More particularly, a run length limited code modulation system is preferably used in recording a PCM audio signal as it permits the minimum transition interval T.sub.min, that is, the minimum interval from one transition between the data "0" and "1" to the next transition between the data "0" and "1", to be extended or lengthened for enhancing the recording efficiency, and further in that it permits the maximum transition interval T.sub.max to be shortened for facilitating the self clocking of the playback or reproducing apparatus on the basis of data derived from the reproduced signal. Further, when the run length limited code modulating system is used, each frame of the recorded signal includes a frame synchronizing signal in the form of a bit pattern corresponding to two successive maximum transition intervals T.sub.max or two successive minimum transition intervals T.sub.min. Since the normal modulation of the recorded signal does not result in maximum transition intervals T.sub.max or minimum transition interval T.sub.min occurring twice in succession, each frame synchronizing signal can be readily identified in the reproduced output. Moreover, in the event of asymmetry, opposite polarity portions of the output signal from the comparator acting as a waveform converting circuit, and which correspond to the two successive transition intervals constituting a frame synchronizing signal, become unequal in duration. In the arrangements shown in U.S. Pat. No. 4,397,011 and 4,418,406, such inequality is detected and gives rise to a change in the threshold level or reference voltage applied to the comparator in the sense to make equal the transition intervals constituting the frame synchronizing signal in the reproduced signal, whereby to compensate for, or correct the asymmetry.
Further, in U.S. Pat. No. 4,397,011, the transition interval of the reproduced frame synchronizing signal is compared with a reference value which corresponds to the desired constant linear velocity of the record disc so that any change in the transition interval of the reproduced frame synchronizing signal due to deviation of the linear velocity of the disc from the desired value is detected as a comparison output which can be used as a velocity servo control signal effective in a motor control circuit to restore the rotated record disc to the desired constant linear velocity.
It has also been disclosed in U.S. Pat. No. 4,397,011, to separate, from the output of the comparator acting as a waveform converting circuit for the reproduced signal, a clock component of the reproduced PCM signal, and to apply such clock component to a phase locked loop (PLL) circuit which produces a reproducing clock pulse of bit frequency having the same time-base fluctuations as the reproduced signal. Such reproduced clock pulse is phase compared with a suitable reference phase signal to produce a corresponding phase control signal by which a predetermined phase of the reproduced PCM signal can be maintained while the record disc is driven at the predetermined constant linear velocity. However, in the foregoing arrangement, the phase servo circuit is made operative only after the velocity servo circuit has been effective to establish the constant predetermined linear velocity of the record disc. The foregoing results from the fact that the PLL circuit of the phase servo circuit has a limited lock range. Thus, following any substantial change in the rotational or angular velocity of the record disc in consequence of a change in the radial position on the disc at which the transducer or pick-up scans the same, the rotation of the record disc can be phase-locked to the reference phase signal only after the velocity servo circuit has been effective to restore or capture the desired constant linear velocity.
Since the above described apparatus has its velocity servo circuit and phase servo circuit acting successively in two steps, only one of such servo circuits can act effectively at any time. Further, in such apparatus, the circuit for detecting the maximum or minimum transition interval for the velocity servo circuit and for the circuit for correcting asymmetry is duplicated and, as a result thereof, the circuits are undesirably complex and expensive.